We are hosting our site on an Ec2 instance. Our Elastic IP is w.x.y.z and Public DNS is:
ec2-w-x-y-z.compute-1.amazonaws.com.

We've bought a domain name domainname.com from a lesser known domain-name-seller. We added an A-record pointing domainname.com to w.x.y.z. Will this work or do we need a CNAME record to point to the same too?

We wanted to use Google apps for emailing so adjusted the TXT/MX records according to the Google Apps ions to be able to send/recv email using @domainname.com email addresses.

Have we got it right, more important, we came across queries relating to email sent from ec2-w-x-y-z.compute-1.amazonaws.com (our users can send email from their onsite accounts) going to spam (rDNS not pointing to domainname.com but to ec2-w-x-y-z.compute-1.amazonaws.com). How can we fix this? We came across SPF records, do they provide a complete solution? We aren't sure as to how to use them. Can you help pls?

Incidentally figured 2 things:

1) CNAME points 1 domain name to another, domain to ip mapping happens in an A record, is this right? So an A record with domainname.com pointing to w.x.y.z does it for us.

2) openspf.org helped us create the TXT record we needed to add to our DNS system to reduce mail originating from either our Google-apps email (MX) or our site (A). Hopefully, rDNS pointing to ec2-w-x-y-z.compute-1.amazonaws.com won't result in much mail going to spam or will it?

Incidentally figured 2 things: 1) CNAME points 1 domain name to another, domain to ip mapping happens in an A record, is this right? So an A record with domainname.com pointing to w.x.y.z does it for us. 2) openspf.org helped us create the TXT record we needed to add to our DNS system to reduce mail originating from either our Google-apps email (MX) or our site (A). Hopefully, rDNS pointing to ec2-w-x-y-z.compute-1.amazonaws.com won't result in much mail going to spam or will it?
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SidFeb 21 '11 at 10:36

2) Almost all blocks of amazon's ip addresses are blacklisted or they will.
This is why Amazon launched Amazon SES (Simple Email Service) and you should use it to fix your issue or using smtp services like sendgrid.com